1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, a secondary battery including a safety vent that can prevent defective fracture, such as a leak, from being generated by an external impact, and that may be rapidly fractured so as to improve stability of the battery when an inner pressure of the battery is increased above a selected value.
2. Description of the Prior Art
With the use increase of portable electronic devices such as cellular phones, portable computers and portable personal digital assistants (PDAs), and the lightweight trend of electric bicycles, electric cars, and electric tools, the importance of a battery used for a power supply by the electronic products have become paramount, thereby driving much research on the design of batteries.
The secondary battery especially can be recharged and minimized, and have large capacity. Thus, a nickel-hydrogen (Ni-MH) battery and a lithium (Li) battery are widely used. Further, with the high performance and supply expansion of the portable electronic devices, the market for a small secondary battery has been expanded.
The lithium secondary battery has an operating voltage of 3.6 V that is three times more than the nickel-cadmium (Ni—Cd) battery or the nickel-hydrogen (Ni-MH) battery that are usually used as a power supply for portable electronic devices. In addition, the lithium secondary battery has a higher energy density per unit weight. Accordingly, the lithium secondary battery has been increasingly used in portable radio frequency devices as well as in other engineering fields.
The lithium secondary battery uses lithium group oxides as their cathode active material, and carbon material as their anode active material. Generally, the lithium secondary battery is divided into a liquid electrolyte battery and a polymer electrolyte battery, depending on a kind of an electrolyte. The lithium secondary battery has been fabricated in various types, such as a cylinder type, an angled type and a pouch type.
When the lithium secondary battery is overcharged, decomposition of the electrolyte occurs at the cathode, and that lithium metal is extracted at the anode. Thus, the battery may be degraded, and heat generation or firing in the battery may take place. The lithium secondary battery may generate gas inside the battery by an electrochemical reaction, and thus the pressure inside the battery may increase and the lithium secondary battery expands and becomes swollen. When the battery is under abnormal conditions, such as the occurrence of an internal short circuit or an overcharge of the battery, the lithium secondary battery may over-expand due to the increase of the inner pressure, and thus the battery may be fractured to fatally affect the stability of the battery.
Accordingly, the lithium secondary battery uses a safety vent to emit the gas generated inside of the battery to the atmosphere when the inner pressure of the secondary battery raises above a selected value. The safety vent may be made by a mechanical method, an etching method or an electric molding method. The safety vent is formed by making a groove with a certain depth in a portion of a cap plate or a can of the lithium secondary battery.
When a contemporary safety vent was formed in the cap plate or the can of the lithium secondary battery, however, the manufacturing process customarily used for forming the groove with a certain depth in each battery is difficult. Accordingly, operating pressure of the safety vent formed in the lithium secondary battery may not be uniform, so the credibility of the safety vent and the secondary battery including the safety vent has been declined.
Therefore, the safety vent may not be rapidly fractured even when the inner pressure of the battery raises above a certain value, or may be undesirably and defectively fractured in a leak type by an external impact generated when the battery is dropped.